A fuel cell system is increasingly being used as a power source in a wide variety of applications. The fuel cell system has been proposed for use in vehicles as a replacement for internal combustion engines, for example. The fuel cell system may also be used as a stationary electric power plant in buildings and residences, portable power in video cameras, computers, and the like. Typically, the fuel cell system includes a plurality of fuel cells arranged in a fuel cell stack to generate electricity, which is used to charge batteries or provide power to an electric motor.
A typical fuel cell is known as a polymer electrolyte membrane (PEM) fuel cell, which combines a fuel such as hydrogen and an oxidant such as oxygen to produce electricity and water. The oxygen is generally supplied by an air stream. In order to perform within a desired efficiency range, a sufficient humidification of the polymer electrolyte membranes of the fuel cell should be maintained. The sufficient humidification desirably extends the useful life of the electrolyte membranes in the fuel cell, as well as maintains the desired efficiency of operation.
As part of the fuel cell system, a water vapor transfer (WVT) assembly may be employed to humidify the air stream entering the fuel cell stack. The WVT assembly transfers water vapor from an exhaust stream from the fuel cell stack to a feed stream entering the fuel cell stack. This is generally accomplished by using a water vapor transfer membrane which allows only water vapor to pass therethrough. This membrane is typically permanently attached to a diffusion media layer. The diffusion media layer is part of a separator that controls gas flow. For example, the separator may be comprised of two of the diffusion media layers separated by strings that define flow channels for gas flow in the separator.
An exemplary WVT assembly is described in Assignee's copending U.S. patent application Ser. No. 12/796,320 to Martinchek et al., the entire disclosure of which is hereby incorporated herein by reference. The WVT assembly includes a plurality of wet plates configured to receive a wet stream and a plurality of dry plates configured to receive a dry stream. The wet plates and the dry plates alternate in a stack and are separated from one another by water transfer membranes. The water vapor transfer assembly permits a transfer of water from the wet stream to the dry stream. The water vapor transfer assembly is disposed between a pair of end plates. The end plates each have a plurality of outwardly extending ribs.
As part of a fuel cell system, the WVT assembly can be used to humidify an air stream entering the fuel cell stack. It is known to assemble the WVT assembly within a housing having a pair of wet stream apertures and a pair of dry stream apertures, and to incorporate the WVT assembly into a fuel cell module such as a lower end unit (LEU) of the fuel cell system. The WVT assembly and the end plates are disposed within the housing. The wet stream apertures are in communication with the wet plates of the WVT assembly and the dry stream apertures are in communication with the dry plates of the WVT assembly. The housing further includes a plurality of channels formed adjacent the dry stream apertures and in fluid communication with the wet stream apertures. The outwardly extending ribs of the end plates cooperate with the channels to define a tortuous bypass flow path between the wet stream apertures of the housing.
The dry stream and wet stream are kept separate using the membranes of the WVT assembly, which allow water vapor to pass from the wet stream to the dry stream without allowing gases such as oxygen through (i.e. the dry air flow is humidified but the oxygen content is not depleted). It is known that a water transfer rate of the WVT assembly degrades over the lifetime of the WVT assembly. It is desirable that a difference between a beginning of life (BOL) water transfer rate and an end of life (EOL) water transfer rate at high flows does not exceed a specific deviation, for example, about −20%, from a target water transfer rate. Due to performance variation between different WVT assemblies, certain ones of the different WVT assemblies may undesirably be closer to the EOL water transfer rate than other ones of the different WVT assemblies at BOL. It is known to mask portions of the plates in the WVT assembly during pre-assembly testing to tune the WVT assembly to a desired BOL water transfer rate. However, the masking is undesirably wasteful since an entirety of the WVT assembly is not being utilized.
It has also been known to use a non-integrated actively controlled bypass valve to control water transfer rate in operation. However, actively controlled bypass valves add significant complexity to the fuel cell system, in addition to requiring significant packaging volume for the associated valve and manifolding.
There is a continuing need for a WVT assembly that provides an ability to tune a BOL water transfer rate at high flows, and minimize BOL performance variation to permit for additional allowable water transfer rate degradation over the lifetime of the WVT assembly. Desirably, the WVT assembly minimizes flow bypass at low flows that would otherwise create poor idle performance, and can be packaged within an existing WVT housing.